This invention relates to a low noise amplifier circuit, suitable for use as the initial stage amplifier in a regenerative repeater used in a digital transmission system. A digital transmission system, which transmits digital signals using, e.g., coaxial cable or balanced cable as a transmission path, provides regenerative repeaters periodically along the transmission path, to regenerate and repeat the signal pulses which have been attenuated or distorted over the transmission path. The initial stage amplifier of such a repeater is required to have an input impedance which is equal to the characteristic impedance of the transmission line over a broad frequency range, and is also required to have a low noise figure.
FIG. 1 shows a conventional transistor amplifier used in such an input stage. In this circuit, input impedance matching is achieved by the parallel resistance R.sub.F together with a series resistance R.sub.E. Although this circuit attains input impedance matching and a reasonably low noise figure, it does not avoid the following disadvantages:
1. The values of R.sub.F and R.sub.E must be selected in a certain relation in order to attain input impedance matching, and this relation prevents optimization of the noise figure of the circuit.
2. The gain of the initial stage amplifier must be large, in order to reduce the influence of noise on the amplifier in the next stage. However, since the circuit of FIG. 1 has a constant gain for all relevant frequencies, increase of the amplifier gain causes an increased probability of low frequency overload, since the transmission path will tend to attenuate the higher frequencies to a greater degree. Thus, the hazard of low frequency overload precludes increasing the gain beyond a certain point, and the noise figure can therefore be improved by increased gain only up to a certain point.